<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
      .box{
          border: 1px solid #111;
          /* overflow: hidden; */

      }  
      .item{
          float:left;
          width:50%;
         height:100px;
         border:1px solid pink;
         box-sizing: border-box;

      }
      /* inline 行内元素 */
      .box:after {
          content:'';
          display:block ;
          clear:both
          /* 清楚浮动必须是块级元素 */
      }
    </style>
</head>
<body>
    <div class="box">
        <div class="item"></div>
        <div class="item"></div>
        <div class="item"></div>
    </div>
</body>
</html>